This invention relates generally to balancing of rotors, and more particularly concerns method and apparatus for quickly and efficiently determining the need and extent of rotor blade pitch adjustment to alleviate an out-of-track condition, the need for blade substitution, and the amount of weight to be added to or subtracted from predetermined positions on the rotor to achieve dynamic balance. The invention is especially applicable to aircraft lift or propulsion rotors, as for example on helicopters and airplanes.
It is a well known fact that a slight out of balance condition of a helicopter main rotor or tail rotor can cause roughness and vibration leading to premature wear and failure of the machine, and fatique and annoyance of pilot and passengers. In the past, techniques used to correct out-of-balance in the field were crude, time consuming and inaccurate. For example, the mechanic would randomly add a weight to one of the several possible points of weight addition. The pilot and mechanic would hover the ship and try to judge whether the ship felt rougher or smoother; or the same. If rougher, the weight addition would be shifted to an opposite point; if then smoother, more weight would be tried (although it might really require less). If the same, another point would be tried. Generally, this would not lead to a smooth rotor, so weights would be added at another angular position, and the whole operation repeated. By repeating this procedure many times, vibration might be lessened, but the achievement of desired reduction was extremely unusual, because while the pilot could sense vibrational amplitude arising from rotor unbalance, he could not readily sense phase characteristics, and thus had no way of determining the locations for adding weights.
An important solution to the above problems, described in U.S. Pat. No. 3,802,273, concerns the provision of a helicopter main rotor and tail rotor dynamic balancing method on the ship, and apparatus characterized as enabling rapid and accurate balancing under field conditions. That method involves the attachment of an accelerometer to helicopter structure proximate the rotor (main or tail) subject to oscillatory motion induced by rotor dynamic unbalance; operating the accelerometer to produce an electrical signal which is processed to derive a corresponding amplified output signal; triggering a stroboscope in synchronism with cyclic variations in the output signal and directing the stroboscope at the rotor to produce flashes repeatedly illuminating a target rotor blade at a characteristic angularity with respect to its axis of rotation; and, varying the weighting of the rotor as a function of the magnitude of the output signal and the target blade characteristic angularity to achieve substantial balance. The varying step may include establishing a multi-coordinate system, either analog or digital, characterized in that a first coordinate scale delineates peak values of the output signal. A second coordinate scale delineates values of target blade angularity about its axis of rotation, and other coordinate scale means delineates weight valves to be added or subtracted to at least one predetermined position on the rotor, and further characterized in that any point in the system defined by particular coordinates on the first and second scales in turn defines particular weighting (on the other scale means) to be added or subtracted. Such first and second scales may define a polar coordinate system, and the other scale means may include two linear scales respectively associated with two sets of symmetric positions on the rotor, where weights are to be added or subtracted.
The method described in that patent contemplates preliminarily adjusting the rotor blades to accurately track one another during rotary travel; however, such prior adjustment may not always be possible, in practice, and in such event it is sometimes found that main rotor dynamic unbalance is affected by an out-of-track condition to an extent making satisfactory dynamic balance difficult to achieve. Further, main rotor blade out-of-track and out-of-balance conditions affect one another, i.e. interact, during flight, so that mixed, resultant vibrations are produced which are difficult to isolate and eliminate.